The invention relates to a device for amplifying signals comprising at least two amplifier circuits, which device also includes means for precluding interference between the amplifier circuits.
Such a device is known from GB-A 2 134 735. It is known that the amplifier circuits incorporated in such a device may influence each other. This influence, also referred to as interference, may lead to undesirable disturbances in the signals amplified by the amplifier circuits. Interference may occur, for example, if two amplifier circuits are operated at different frequencies. As a result, interference signals may be generated in the amplifier circuits, the frequencies of said interference signals being equal to the sum of or the difference between the original frequencies. Interference may also occur if the amplifier circuits are embodied so as to be switching amplifiers. In this case, switching of a first amplifier may cause a switching action in a second amplifier.
The device known from said English patent application comprises first and second pulse-width modulating amplifier circuits, which both include a comparator and a switching amplifier connected thereto. In these amplifier circuits, an input signal to be amplified is converted into a pulse-width modulated square-wave switching signal by comparing the input signal in the comparator to a triangular reference signal. The switching signal is subsequently amplified by the switching amplifier.
In the known device, interference between the two amplifier circuits as a result of the simultaneous switching of the switching amplifiers is precluded by employing reference signals having different phase. A first reference signal is generated by an oscillator and supplied to the first amplifier circuit. A second reference signal is derived from this first reference signal in a phase shifter, the phase of said second reference signal being shifted ninety degrees relative to the first reference signal. This second reference signal is supplied to the second amplifier circuit. As the phases of the reference signals are different, the phases of the switching signals are different too. By virtue thereof, the switching amplifiers cannot be switched simultaneously.
The known device comprises relatively many parts, so that the device is relatively complicated and expensive.